1) Field Of The Invention
The invention relates to a process for pretreating the surface of a molding by means of an electrical corona discharge which takes place between high voltage electrodes and a counter-electrode, between which the molding passes, and an apparatus for pretreating the surface of the molding. The present invention is particularly suitable for moldings in the form of films having a thickness greater than 1 mm.
In many cases, the normally smooth surface of molded plastic materials, in particular of films, presents difficulties in that the film webs have extremely good slip and readily tend to telescope when being wound on a roll. Further difficulties arise during treatment of these materials to increase the adhesion of printing inks, coatings, adhesives, metals applied by vapor deposition, and the like, due to the smooth surfaces of films or molded materials. The prior art for overcoming these difficulties includes physicochemical surface modification of plastics, in particular of films. A basic process which only produces changes on the surface of the plastic consists of pretreating the surface of the plastic by means of an electrical corona discharge.
According to German Offenlegungsschrift 3,247,795, corona pretreatment of a film web of plastic is carried out by a procedure in which the upper surface and/or the lower surface of the film web are exposed to a corona which results in different pretreatment intensities. For this purpose, the film web to be treated is fed over an electrically grounded roller, and electrical charging is effected by exposure of one or both surfaces of the film web to an electrical corona discharge. The electrical corona discharge is generated by applying a high-frequency alternating current at high voltage to an electrode spaced a small distance away from the roller. The pretreatment is carried out in general in air under atmospheric pressure.
The constantly increasing market requirements of products having improved surface properties have also led to the development of processes using chemically reactive substances which, for example, cleave certain chemical bonds in the surface and thus alter the surface properties of plastics. U.S. Pat. No. 3,142,630 describes a process for increasing the adhesion, in which a film web is passed through a non-ionizing liquid and is exposed to a corona discharge in the liquid. This liquid may be, for example, a transformer cooling oil, vegetable oil or another pure oil, which is free from impurities and which is substantially electrically nonconducting.
British Patent No. 938,325 describes a process for the pretreatment of thermoplastic films, in which an electrical corona discharge takes place at the surface in a nitrogen atmosphere. The nitrogen is passed via distribution lines, through hollow electrode lines, into the corona discharge zone.
In the arrangement described in U.S. Pat. No. 3,274,089, organic compounds from the group consisting of polymerizable organic compounds, non-polymerizable organic compounds having substitutable hydrogen atoms and perhalogenated hydrogen are passed into the corona discharge zone through distribution lines in order to modify the surface of film webs or articles made of polymers.
The common feature of these known processes is that reactive gases are passed into the corona discharge region between the electrodes, or the corona discharge is allowed to take place in a non-conducting liquid.
Japanese Patent No. 17 747/73 discloses an apparatus in which a film surface is exposed to a corona discharge. The electrode connected to the generator consists of porous sintered metals and of a plurality of metal nets. The high voltage discharge electrode is shaped in such a way that liquid fed into it accumulates and is stored. As a result of the voltage applied to the discharge electrode, the stored liquid is converted into the gas phase and emerges from the porous sintered metals in the form of gas particles, which move toward the film surface under the influence of the electric field lines of the corona discharge.
In the apparatuses and processes which include a liquid as the discharge electrode in the corona discharge process, it is necessary to rely on special apparatuses which permit storage or accumulation of the liquid and furthermore must consist of a material which allows the liquid converted into the gas phase to pass there through into the corona discharge zone. If the film web to be pretreated is passed through a liquid in which the corona discharge takes place, the transport velocity of the film web through the liquid is obviously limited. If a corona discharge is effected in a reactive atmosphere on the surface of the plastic, various layers can be subsequently applied by further process measures to treat the surface of the plastic. Coating simultaneously with the pretreatment is not possible in such a case. The same applies to pretreatment of surfaces of plastics where the corona discharge is carried out on the surface in a liquid.
German Offenlegungsschrift No. 3,705,482 describes a process for the physicochemical pretreatment of the surface of moldings of plastics, in which controlled reaction mechanisms are triggered on the treated substrates by introducing liquids atomized to yield aerosols into the corona discharge. In the apparatus, the substrate to be treated is passed through a discharge gap which is formed by a roller at ground potential having a dielectric covering, as the counter-electrode, and electrodes at high voltage. The aerosol is blown into the discharge gap from a separate atomizing apparatus by means of a carrier gas. The high voltage corona generators which are available on the market and which usually produce output voltages between 5 and 25 kV are suitable for generating the high voltage required for the corona discharge. These high voltages are sufficient for striking a corona discharge in the discharge gap which is about 1.5 to 2 mm wide, and for pretreating the flexible substrates which are not more than 500 microns (0.5 mm) thick. However, these generators cannot be used for corona treatment of thick plastic sheets or moldings in the range from 1 to 60 mm. In order to be able to strike any homogeneous corona discharge at all in such large discharge gaps, extremely high voltages, which can be delivered only by specially designed generators, are required.
Owing to their chemical composition, many materials are electrically non-conducting or electrically conducting to a virtually immeasurable extent and are therefore generally regarded as insulators. However, the low conductivity of these materials is the reason why they often accumulate very high electrostatic charges at their surface which, particularly in the case of sheet-like polymer products such as disks, may have many disadvantageous effects and even make the product unusable and are therefore very undesirable. The charges are formed during production, further processing, or using the disks wherever frictional activity take place. For example, where two materials come into contact with one another and then separate again, such as at rollers during production, and they can lead to production problems, and accumulation of dust, and can influence measuring and control instruments, and can even cause explosions. High static charges (more than 5,000 V) can ignite gas/air mixtures produced by evaporating solvents, and low static charges of less than 1,000 V can optically influence or destroy the surface of film by formation of stripes and accumulation of dust and dirt.
The magnitude of the electrostatic charge depends on the electrical conductivity of the materials. Good conductors are, for example, metals, carbon black, graphite and polyenes, which lose their charge immediately; while poor conductors, in particular polymers, maintain their charge over prolonged periods, namely for seconds to minutes and even several days.
The problem with particles of plastics is generally their poor antistatic properties. Highly electrostatically charged particles of plastics present considerable difficulties during subsequent treatment steps, such as, for example, coating or printing. Regarding the printing of expanded polystyrene sheets by the screen printing technique, it is known, for example, that the electrostatic charges present on the sheet eject the printing ink filling the screen from the screen mesh, thus making printing impossible.